The present invention is directed to an improved method of forming hydraulic cement powder product using a high pressure roll press.
Hydraulic cements, such as portland cement, are prepared by sintering a mixture of components which normally include calcium carbonate (as limestone), aluminum silicate (as clay or shale), silicon dioxide (as sand), and miscellaneous iron oxides. During the sintering process, chemical reactions take place to produce hardened nodular products, commonly called clinkers. The clinkers are composed of the reaction products of calcium oxide with acidic components which primarily provide tricalcium silicate (C.sub.3 S), dicalcium silicate (C.sub.2 S), tricalcium aluminate (C.sub.3 A) and a ferrite solid solution phase approximating tetracalcium aluminoferrite (C.sub.4 AF). The specifics of the raw materials are adjusted to produce clinkers having the desired composition for cement powder formation. Once the clinker material has cooled, it is pulverized, normally with a small amount of gypsum, to provide a fine, homogeneous powdery hydraulic cement product.
The conventional means of producing cement powder is to pulverize a clinker/gypsum mixture using a ball mill. The clinker and gypsum are introduced into a tubular ball mill. Due to the nature of a ball mill, the materials contained in the mill are continuously subjected to impact forces. Such forces aid in churning-up the powdery product and do not permit the powder to combine into large agglomerates. The powder leaving the ball mill is usually fed into a separator which classifies the material by means of its rate of settling in air; the larger particles, which settle most rapidly, being returned to the ball mill for further grinding, and the smaller particles being collected as finished cement.
Due to the inefficiency of action of the ball mill grinding process, a large amount of energy is required to properly mill the material. For example, energy requirements for finish grinding by ball mill may be as high as 75 kWh/ton of product. Part of this energy is used in the continuous impacting of the powdery product prior to its removal from the mill. To further aid in such removal and, thereby, improve on the efficiency of the milling process, a variety of materials commonly known as grinding aids have been used in ball mill operations. Typical grinding aids include glycols, alkanolamines, aromatic acetates and the like. They serve to fluidify the cement powder in the mill and thereby allow more rapid transit of the finer particles out of the system.
Recently, powdery hydraulic cement has been formed from clinker by passing the clinker (alone or with gypsum) through a high pressure roll press. The raw material is introduced, via a hopper, to a set of rollers operated at pressures in the neighborhood of 3000 atm (300 MPa) to exert a squeezing force on the clinker to cause it to fracture into a powder. The powdery product of the roll press is in the form of an agglomerated cake ("press cake"). The press cake is deagglomerated by passing through a breaker device, such as a hammer mill, and then to a classifier which returns coarse material to the press. The coarse material results from either (a) partial breakage of clinker (inefficiency of roll press) or (b) partial breakage of the agglomerated press cake material (inefficiency of breaker device).
Although the high pressure roll press is more efficient in energy consumption with respect to the ball mill process, it is desired to improve this newer method by increasing the efficiency of the press and/or breaker device so that, in a given pass-through, there is a higher amount of desired fine particulate produced. Conversely, there would be less coarse material ejected from the classifier and returned to the roll press.